A sensor for detecting a target substance in a biological sample has been proposed in the past. With a blood glucose sensor, which is an example of a sensor, the biological sample is blood, and the target substance is glucose.
Most of the blood glucose sensors that have been proposed are electrochemical blood glucose sensors. An electrochemical blood glucose sensor comprises an enzyme and a mediator. This enzyme oxidizes glucose by specifically reacting with the glucose in blood. The mediator accepts electrons generated by oxidation. The mediator that has accepted these electrons is electrochemically oxidized by electrodes, for example. The glucose concentration in the blood, that is, the blood glucose level, is easily detected from the amount of current obtained by this oxidation.
In the past, potassium ferricyanide has usually been used as the mediator in the above-mentioned type of electrochemical blood glucose sensors (see Patent Literature 1, for example). Potassium ferricyanide is chemically stable in a dry state at room temperature, and is also low in cost. Furthermore, potassium ferricyanide has high solubility in samples whose solvent is water, such as blood. Thus, potassium ferricyanide is particularly favorable with certain sensors (those in which an enzyme and a mediator are actively dissolved in blood during blood glucose detection).
The ferricyanide ions contained in potassium ferricyanide dissolve quickly in blood, accept electrons from the enzyme that has reacted with glucose, and become ferrocyanide ions. These ions are electrochemically oxidized by electrodes, and produce current corresponding to the blood glucose level.
However, with a blood glucose sensor in which potassium ferricyanide is used as a mediator, a problem was that measurement error was caused by other substances present in the blood. This measurement error occurs as follows. Ascorbic acid (vitamin C) and other such substances are present along with glucose in blood. Ascorbic acid is oxidized by the electrodes in the blood glucose sensor along with the ferrocyanide ions. As a result, current originating in ascorbic acid is superimposed with current originating in the blood glucose level, and the resulting current value is detected as current expressing the blood glucose level. This is what causes measurement error.
This measurement error occurs because the potential of the electrodes needed to oxidize the ferrocyanide ions is significantly higher (positive) than the potential for oxidizing ascorbic acid. Specifically, the oxidation potential of the ferrocyanide ions themselves (approximately 160 mV vs. Ag|AgCl) is far higher than that of ascorbic acid (approximately −140 mV vs. Ag|AgCl), so a large measurement error results.